Navigation equipment mounted in a vehicle carries out display of the vehicle's position on a road shown in a map, guidance, etc. When displaying the vehicle's position on an on-screen road, the navigation equipment measures the movement of the vehicle by using the GPS and an dead reckoning sensor including a velocity sensor and an angular velocity sensor, and performs a map matching process so as to determine the position of the vehicle on a road link shown by map data. However, the dead reckoning sensor has a rate (referred to as a “Scale Factor”) with which the dead reckoning sensor converts pulses outputted from the velocity sensor into a distance and the rate differs for every vehicle. Furthermore, because the bias voltage (referred to as the “offset”) of the angular velocity sensor drifts according to temperature, it is necessary to always check occurrence of an error and to correct this error properly.
In addition, the GPS has problems when used as follows.
(1) The GPS positioning requires radio waves from three or more GPS satellites in principle. However, when the field of vision over the vehicle becomes narrowed because of buildings or the like around the road along which the vehicle is traveling, the radio wave from one or more GPS satellites is blocked. When the number of GPS satellites which can be used for the positioning becomes smaller than three due to this blocking, the navigation equipment cannot carry out the positioning and the availability of the navigation equipment is reduced. Hereafter, this problem is referred to as an “availability problem”.
(2) When a building or the like exists around the road along which the vehicle is traveling, the radio wave from a GPS satellite is reflected by the building or the like and propagates through a plurality of paths (a multipath) until the radio wave from the GPS satellite reaches the GPS receiver on the ground, and therefore the propagation delay time of the radio wave becomes long. In the GPS positioning, because the vehicle's position is measured from a pseudo range which is acquired by converting the propagation delay time into a distance, the occurrence of multipath propagation provides a large error for the vehicle's position. Hereafter, this problem is called a “multipath problem”.
As mentioned above, a problem with the case of carrying out the GPS positioning in an urban area is that the availability is reduced, and multipath propagation occurs and this results in a large error occurring in the vehicle's position. In a conventional navigation equipment, in order to eliminate this problem resulting from the GPS positioning, correction of the vehicle's position on a road link by using measurement data acquired from a dead reckoning sensor is carried out.
As such a technology, patent reference 1 discloses a GPS receiver which uses a Kalman filter in order to improve the accuracy of positioning computation results, and a car navigation system. The GPS receiver and the car navigation system disclosed by this patent reference 1 are constructed in such a way as to return the results of map matching to the Kalman filter in order to guide the convergence of the Kalman filter used for the GPS positioning arithmetic operation toward a correct direction. This is based on that the results of carrying out map matching using measurement data acquired from a dead reckoning sensor is more accurate than that provided by the results of carrying out map matching using GPS positioning data.
Furthermore, patent reference 2 discloses a GPS receiver with a DR (Dead Reckoning) function. The GPS receiver disclosed by this patent reference 2 is constructed in such a way as to, for example, feed the results of map matching carried out by navigation equipment back to the GPS receiver itself so as to cancel a positioning error. The GPS receiver with a DR function performs DR positioning and GPS positioning independently, but does not complexly carry out positioning by using a Kalman filter or the like.
Furthermore, patent reference 3 discloses a GPS receiver that reduces a pseudo range error without causing a reduction in the availability, thereby being able to improve the positioning accuracy. The GPS receiver disclosed by this patent reference 3 is constructed in such a way as to smooth the pseudo range (measured from the propagation delay time) which is easy to be affected by the influence of multipath propagation by using a range rate (measured from a Doppler shift of the carrier frequency) which cannot be easily affected by the influence of multipath propagation. Even in a case in which instantaneous interception of the radio wave from a GPS satellite occurs, the GPS receiver interpolates the pseudo range by using the range rate estimated from a relative movement between the GPS satellite and the vehicle, and uses the pseudo range for positioning calculation. Thereby, the GPS receiver resolves the multipath problem and the availability problem.    [Patent reference 1] JP, 2001-272239,A    [Patent reference 2] JP, 2002-213979,A    [Patent reference 3] JP, 2006-322846,A
By the way, navigation equipment that performs positioning and map matching by using a GPS receiver and a dead reckoning sensor has to satisfy the following minimum requirements: being able to receive radio waves from three or more GPS satellites existing over the vehicle, and the GPS receiver being able to carry out either two-dimensional positioning or three-dimensional positioning, in order to use the results of GPS positioning.
However, when the vehicle is traveling along a road in an urban area or the like where buildings or the like extends along the road, the field of vision over the vehicle becomes narrowed because of the buildings or the like and, if the number of GPS satellites from which the GPS receiver can receive radio waves becomes smaller than three, there arises an availability problem that the availability of the GPS decreases. Furthermore, there arises a multipath problem that the GPS receiver often receives radio waves reflected by buildings or the like at such a location and easily produces GPS positioning results having a large error. To solve this problem, a conventional navigation equipment is constructed in such a way as to be able to determine (dead reckoning) the position of the vehicle by using the travelled distance and turning angle of the vehicle (user) which are measured by a dead reckoning sensor so as to correct the position of the vehicle on a road link or the like.
However, in a case in which the vehicle starts traveling from a place outside roads, such as a multi-level car parking tower, large errors occurring in the position and heading of the vehicle which are determined through dead reckoning cause a mismatch of the position of the vehicle onto an incorrect road link at a time when the vehicle travels along a road after that. In this case, when the navigation equipment is in a state in which it cannot carry out GPS positioning, the mismatching continues. When the current position of the vehicle is not correct, the navigation equipment makes a mistake in the route guidance etc.
To solve these problems, in accordance with the technology disclosed by patent reference 1, the results of carrying out map matching by using measurement data acquired from the dead reckoning sensor are fed back to the Kalman filter on the basis of the premise that its accuracy is higher than that provided by the results of carrying out map matching by using GPS positioning data so as to guide the direction of the convergence of the Kalman filter which is used for the GPS positioning arithmetic operations. However, as mentioned above, when the results of carrying out map matching are not correct, a disadvantage is that the results of mismatching obstruct normal GPS positioning, and therefore the identification of the position of the vehicle on a correct road link is hindered.
Furthermore, to solve the above-mentioned problems, in accordance with the technology disclosed by patent reference 2, attempt is made to remove errors in dead reckoning by using the results of map matching. However, this technology is effective when the map matching can be carried out correctly, like the technology disclosed by above-mentioned patent reference 1, though in a state in which a mismatch occurs before the GPS positioning is carried out, desired advantages cannot be provided, as mentioned above.
In addition, to solve the above-mentioned problems, in accordance with the technology disclosed by patent reference 3, the pseudo range which is easy to be affected by the influence of multipath propagation is smoothed by using the range rate which cannot be easily affected by the influence of multipath propagation so as to reduce the influence of multipath propagation. Furthermore, when instantaneous interception of the radio wave from a GPS satellite occurs, the GPS receiver interpolates the pseudo range by using the range rate estimated from a relative movement between the GPS satellite and the vehicle, and uses the pseudo range for the positioning calculation. Thereby, the reduction in the availability and the influence of multipath propagation can be reduced. However, when the number of radio waves from GPS satellites which can be used for the positioning after the navigation equipment is powered on is smaller than three, because even this technology does not make it possible to carry out the GPS positioning, the desired advantages cannot be provided.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide navigation equipment that can reduce the influence of multipath propagation without causing any reduction in the availability thereof, thereby being able to improve the positioning accuracy.